At present, due to a limitation of a Shannon capacity, a conventional single-input single-output (SISO, single input single output) antenna system cannot meet requirements for a large capacity, a high rate, and high reliability of a new generation wireless communications system. In view of the objective fact that spectrum resources are limited, how to achieve higher spectrum utilization has become a problem that urgently needs to be resolved in development of new technologies in the current wireless communications field. In a multiple-input multiple-output (MIMO, Multiple-Input Multiple-Output) antenna system, a communications link can be effectively divided into a plurality of parallel subchannels, thereby greatly improving a channel capacity, removing a limitation of the Shannon theorem, and greatly improving reliability.
However, when a multiple-input multiple-output (MIMO, Multiple-Input Multiple-Output) antenna system is applied to a base station, because available space of the base station is relatively large, a multiple-antenna technology can be easily applied. For terminal devices that are increasingly miniaturized, a plurality of antennas need to be centralized in small space, and to achieve good performance, the antenna modules need to be well isolated, and a low correlation coefficient is required for the antenna modules. In addition, at present, on a worldwide basis, there are a plurality of standards to meet different applications, and these standards cover different bands. Therefore, an antenna system needs to be capable of operating in a plurality of bands. Space in a handheld device (such as a mobile phone) is very limited, and a distance between antenna modules forming an MIMO antenna is very short. Consequently, it is very difficult to design a MIMO antenna system that meets these requirements and has good performance.